This invention relates to an alignment device and, more particularly, to an alignment device for aligning a reticle with a wafer in the manufacture of semiconductor circuit devices. The invention also relates to an exposure apparatus for use in the manufacture of semiconductor circuit devices, including an alignment device for aligning a reticle with a wafer.
Recent trends of larger capacities of the semiconductor circuit devices, such as integrated circuits, have forced further miniaturization of the circuit patterns. The requirement of further miniaturization can be met by using a shorter wavelength of light for the exposure of semiconductor substrates. For example, in a case of proximity exposure, the resolvable line width is decreased in proportion to the root of the wavelength used, while in a case of projection exposure, the resolvable line width is decreased in proportion to the wavelength used. In view of this, it has been proposed to use a high-power laser such as an excimer laser, as a light source for the exposure of the semiconductor substrates.
The reduction of the resolvable line width of the circuit pattern, as described above, necessarily requires improvements in the accuracy of alignment between the reticle and wafer upon transfer of the pattern of the reticle onto the wafer.
In order to achieve higher alignment accuracies, in a case where the alignment is effected by means of a light beam of a wavelength within a visible range, it would be necessary to make larger the numerical aperture (NA) of the alignment optical system to allow the alignment optical system to take up, among the diffractively reflected light rays from the alignment mark, the light rays of the diffraction orders from low to higher. By this, the output level of alignment signals will be increased, so that higher alignment accuracies will be attainable. However, this requires a substantial structural change of the alignment optical system.
Examples of the wavelengths conventionally used for the alignment beam in the visible range are a wavelength 546 nm (Hg lamp), a wavelength 633 nm (He-Ne laser), a shorter wavelength 436 nm (Hg lamp), a wavelength 405 nm (Hg lamp), a wavelength 442 nm (He-Cd laser), etc. With the alignment beam as above, manual alignment through a microscopic observation system and/or auto-alignment through a TV camera are carried out.
If, on the other hand, an alignment beam in the ultraviolet (UV) range is used to improve the alignment accuracies, the manual alignment through the microscopic observation system is nullified because the ultraviolet light is out of the visible range. Further, ordinary TV cameras do not have sensitivities to the ultraviolet range, so that a special image pickup device is required to achieve the auto-alignment.
When, on the other hand, the alignment is to be effected by a laser beam having a wavelength which is at an end portion of the visible range on the shorter wavelength side, such as the laser beam of the He-Cd laser, it is necessary for the manual alignment through the microscopic observation system to amplify the intensity of the alignment beam because the sensitivity of human eyes to the He-Cd laser is poor.